Anyone who has waded through a sea of vegetation looking for ancient hut circles or a burial cist will be very aware of the way in which bracken hides surface archaeological remains. But bracken (Pteridium aquilinum) does far more damage underground.
Unlike most ferns, bracken does not rely on spores to spread. Instead it depend mainly on the growth of its underground rhizomes, which allow an invading bracken fron to move at a rate of up to 3m per year. Within a stand of bracken, the number of fronds can be high – up to 100 per square metre; but the weight of the underground rhizomes is usually two or three times more than that of the fronds , at up to 15 tons per acre (40 tonnes/ha).
Excavation of a prehistoric roundhouse by the Dartmoor Archaeology and Bracken Project directed by Sandy Gerard, showed that there were over 7km of rhizomes inside just on house; these occupied 10% or more of the soild volume at the most affected levels, displaying finds and destroying stratigraphy.
The longer bracken covers a site, the more this will happen, until stratigraphy can no longer be read and finds have no meaningful context. The damage is greatest at fairly shallow levels (less than half a metre) but bracken rhizomes can go at least twice as deep.
The scale of the problem is large and increasing. Over 3,000 square kilometres of hte UK were recorded as affected in 1978, and over 4,000 square kilometres 20 years later, probably mainly as a result of reduced grazing. This is nearly 2% of this country’s land area. And bracken undoubtedly affects a lerge number of upland archaeological sites, as it likes the deeper and better drained soils that early farmers preferred.
There is as yet no magic formula for bracken control. The most commonly reccommended chemical treatment. Asulox, is fairly effective in the short term, and has been widely used; over 1,000 square kilometres were sprayed between 1980 and 2002. But Asulox is expensive, and unless there is careful follow up, the bracken comes back within a few years. The 2000 countryside survey ahowed that by 1998 bracken had re-establisheed in about three quarters of sprayed areas. Asulox is currently undewrgoing re-evaluation under EU regulations: if not relisted it would not be available after 2011.
Non-chemical treatments – mainly rolling “bashing”, cutting and managed grazing – can be effective, and landowners and managers concerned at the possible environmental effects of chemicals prefer them. However, they only work well if done carefully and often enough, and short-lived clearance followed by regrowth is probably more damaging than leaving established bracken alone.